Wave drag is a phenomenon that occurs as a result of the compression of air as an aircraft approaches the speed of sound. The compression generates a shock wave which may be accompanied by a localized change in the pressure and/or temperature of the air. At subsonic speeds, wave drag forms a relatively small portion of the overall aerodynamic drag on an aircraft. However, wave drag increases significantly as an aircraft approaches Mach 1.
Known methods of minimizing wave drag include designing an aircraft with a configuration that follows the Whitcomb Area Rule. The Whitcomb Area Rule dictates a minimization of change in the cross-sectional area of the aircraft in the longitudinal direction. In this regard, an aircraft following the Whitcomb Area Rule has a relatively smooth or gradual change in the size of the cross-sectional area regardless of changes in the cross-sectional shape. In conventional aircraft, the forward end of an aircraft fuselage may have a relatively small cross-sectional area. Unfortunately, the cross-sectional area may increase significantly and abruptly at the wings and/or engines and which may result in significant wave drag at transonic speeds.
Attempts to minimize changes in the longitudinal cross-sectional area of an aircraft including locally necking down the fuselage at the juncture with the wings to minimize the change in total cross-sectional area at that location. Unfortunately, designing and manufacturing an aircraft with a fuselage having a variable cross-sectional shape adds to the overall cost and complexity of the aircraft. In addition, in a commercial airliner, locally necking down the fuselage at the wings may be economically undesirable due the potential loss of revenue-generating passenger seats or cargo space.
Another approach to minimizing wave drag in an aircraft is by forming the wings in a swept arrangement. Wing sweep may minimize changes in the longitudinal cross-sectional area of an aircraft by distributing the cross-sectional area of the wings over a longer length of the fuselage. Wing sweep may delay the onset of wave drag rise by increasing the Mach number required to generate shockwaves on the wing surface. The increase in Mach number may occur due to alignment of the pressure isobars with the sweep of the wing such that shocks will only form when the component of velocity perpendicular to the pressure isobars reaches sonic speeds. Unfortunately, excessive wing sweep may have an effect on the low speed performance of an aircraft. In addition, wing sweep may add cost and complexity to the aircraft design and manufacturing process.
For aircraft operating at subsonic speeds, wave drag accounts for a relatively small portion of the total aerodynamic drag of the aircraft, as indicated above. However, a small reduction in wave drag may translate into a significant increase in fuel efficiency of the aircraft. Military aircraft that operate in the transonic region may also benefit from a reduction in wave drag with an increase in top speed and/or an increase in range.
As can be seen, there exists a need in the art for an aircraft configuration that minimizes wave drag and which may be provided with minimal impact on the aircraft design and manufacturing process.